When no-load energizing is performed by supplying power to a transformer in a state in which there is residual magnetic flux in the transformer core, a large magnetizing inrush current may flow. Generally, the magnitude of this magnetizing inrush current can be greater than or equal to several times the rated load current of the transformer. Therefore, such a magnetizing inrush current flows, resulting in that the system voltage fluctuates, and when this voltage fluctuation is large, consumers can be affected.
Accordingly, as a method of the prior art for suppressing magnetizing inrush currents, for example, a magnetizing inrush current suppression method is proposed in which a circuit breaker with a resistor, formed by connecting in series an closing resistor and a contact, is connected in parallel with one of two main circuit breaker arranged side by side, and power is turned on to this circuit breaker with a resistor in advance of main contact of the two main circuit breaker (see Patent Document 1).
Further, a method is known in which, when a three-phase transformer of a solidly earthed system is supplied with power using three single-phase circuit breakers, one arbitrary phase is supplied with power in advance, and thereafter the remaining two phases are supplied with power, to suppress magnetizing inrush currents (see Non-patent Document 1).
Patent Document 1: Japanese Patent Application Laid-open No. 2002-75145, “Gas Circuit Breaker with Device for Suppression of Magnetizing Inrush Currents”
Patent Document 2: Japanese Patent No. 3,804,606
Non-patent Document 1: IEEE Trans., Vol. 16, No. 2, 2001, “Elimination of Transformer Inrush Currents by Controlled Switching-Part I: Theoretical Considerations”
In the case of the magnetizing inrush current suppression method disclosed in the above-described Patent Document 1 employing a circuit breaker with a resistor, which is formed by connecting in series an closing resistor and a contact, because it is necessary to specially add a circuit breaker with a resistor to the ordinary circuit breaker, in terms of the circuit breaker as a whole, larger equipment size is undeniable.
Moreover, the magnetizing inrush current suppression method in the above-described Non-patent Document 1 in which a transformer of an effectively grounded system is supplied with power by single-phase type, namely, single-phase circuit breakers, has a drawback that it is impossible to suppress the magnetizing inrush current which occurs to a transformer of a non-solidly earthed system. Specifically, when single-phase circuit breakers supply power to energize a no-load transformer installed in a non-solidly earthed system, because closing of one-phase circuit breaker can not apply voltage to transformer windings, closing of the second and third phases follows the same condition as three-phase simultaneous closing, resulting in that it is impossible to suppress the magnetizing inrush current.
Further, it is essential that, when suppressing magnetizing inrush currents at the time of supplying power of transformer, the magnitude of the residual magnetic flux when the transformer is interrupt be ascertained, from a relation with magnetic saturation of the transformer core. However, as described above, when single-phase circuit breakers supply power to energize a no-load transformer installed in a non-solidly earthed system, if the circuit breakers interrupt at the zero point the magnetizing current flowing in the no-load transformer, after interrupting the first phase a zero-phase voltage appears, and after interrupting the second and third phases the zero-phase voltage becomes a DC voltage and remains on the transformer.
Consequently when the voltage to ground at each of the transformer terminals on the side interrupted by the circuit breakers is being measured, the above-described DC voltage is measured after interrupt. Therefore, the residual magnetic flux in the transformer core can not be accurately calculated by integration of the voltage to ground of each terminal.
For example, FIG. 3 shows a phenomenon that, when a transformer in a non-solidly earthed system is interrupted, a DC voltage occurs in the primary terminal voltage. Especially, FIG. 3 (b) and (c) show the transformer primary voltage to ground and the magnetic flux calculated by integrating the voltage to ground when circuit breakers interrupt the transformer the primary side of which is a Y connection and a neutral point is non-grounded. Further, as shown in FIG. 3 (b), after the circuit breakers interrupt the current, a DC voltage occurs in the transformer primary voltage to ground. The voltage of the Y-connected neutral point is also the same.
Here, if the residual magnetic flux is calculated by integrating the transformer terminal voltages 4 to 6, because the occurred DC voltage is to be calculated, as shown in FIG. 3 (c), the residual magnetic fluxes 33 to 35 of each phase increase as time passes, and finally diverge. In other words, when the magnetic fluxes are calculated by integrating the transformer terminal voltages 4 to 6, the residual magnetic flux can not be accurately calculated.